Sputtering target for production of magnetic recording medium, method for producing magnetic recording medium using same, and magnetic recording medium

ABSTRACT

The present invention provides a sputtering target for production of a magnetic recording medium including at least a nonmagnetic undercoat layer, a magnetic layer, and a protective layer laminated sequentially on a nonmagnetic substrate, the sputtering target being used for film formation of the magnetic layer, the sputtering target comprising a mixture of a metal and an oxide, and the particle diameter of the oxide in the sputtering target being 10 μm or less. The sputtering target suppresses abnormal discharge occurring during film formation of a granular magnetic layer of the magnetic recording medium, and suppresses occurrence of foreign objects on the magnetic recording medium.

[0001] This application is based on Japanese Patent Application No.2000-46471 filed Feb. 23, 2000, the content of which is incorporatedhereinto by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to a sputtering target for production of amagnetic recording medium, which is used for film formation of amagnetic layer of the magnetic recording medium mounted in variousmagnetic recording devices, including external storage devices ofcomputers. The invention also relates to a method for producing amagnetic recording medium by use of the sputtering target, and amagnetic recording medium produced by this method for production.

[0004] Hereinafter, the sputtering target will be referred to simply asa “target”.

[0005] 2. Description of the Related Art

[0006] Various compositions and structures of a magnetic layer andvarious materials for a nonmagnetic undercoat layer have been proposedfor a magnetic recording medium required to have a high recordingdensity and a low noise. In recent years, a proposal has been made for amagnetic layer, generally called a granular magnetic layer, which has astructure comprising magnetic crystal grains surrounded with anonmagnetic nonmetallic substance such as an oxide or a nitride.

[0007] The granular magnetic layer is considered to obtain low noisecharacteristics for the following reason: A nonmagnetic nonmetallicgrain boundary phase physically separates the magnetic grains. Thus, themagnetic interaction between the magnetic grains lowers to suppress theformation of zigzag domain walls occurring in a transition region ofrecording bits.

[0008] Examples using the granular magnetic layer are shown in JapanesePatent Application Laid-open No. 8-255342 (1996) and U.S. Pat. No.5,679,472. Japanese Patent Application Laid-open No. 8-255342 (1996)proposes the achievement of a low noise by laminating a nonmagneticfilm, a ferromagnetic film, and a nonmagnetic film sequentially, andthen heat treating the laminate to form a granular recording layercomprising ferromagnetic crystal grains dispersed in the nonmagneticfilm. In this case, a silicon oxide or nitride is used as thenonmagnetic film.

[0009] U.S. Pat. No. 5,679,473 describes that RF sputtering is performedwith the use of a CoNiPt target incorporating an oxide such as SiO₂,whereby there can be formed a granular recording layer having astructure comprising magnetic crystal grains surrounded by a nonmagneticoxide and thereby individually separated. As a result, a magneticrecording medium having high Hc (coercive force) and a low noise isobtained. The method described in U.S. Pat. No. 5,679,473, for filmformation of a granular magnetic layer by performing RF sputtering usinga CoNiPt target incorporating an oxide such as SiO₂, can produce amagnetic recording medium without involving process such as heattreatment, and is thus excellent in mass producing ability.

[0010] However, when a target having an oxide dispersed in a metal issputtered, abnormal discharge due to charging of oxide particles, as aninsulating material, at the start of sputter discharging tends to occur.When abnormal discharge occurs, the surface of the target is partiallyheated, and thus melts and flies. As a result, oxide particles measuringseveral micrometers may adhere onto a magnetic recording medium disposedat an opposed position.

[0011] If the oxide particles thus adhere onto the magnetic recordingmedium, the incident that the particles collide with the head arises. Toprevent this incident, the particles may be removed by sliding, forexample, a cleaning tape over the medium having the film formed bysputtering. This sliding treatment peels not only the particles, butalso the magnetic layer and a protective layer, causing problems such assignal defects or corrosion.

[0012] Japanese Patent 2948019 describes that the particle diameter ofan insulating substance is set at 20 μm or less in order to resolveabnormal discharge occurring when sputtering is performed using asputtering target comprising a sintered mixture of the insulatingsubstance and a conductive substance. However, this sputtering target isused for film formation of a heating resistor thin film of a thin filmthermal head. It has been unknown whether the sputtering target can beused for film formation of a magnetic layer of a magnetic recordingmedium because of its magnetic characteristics, for example.

SUMMARY OF THE INVENTION

[0013] Extensive studies have been conducted to prevent the foregoingabnormal discharge and adhesion of oxide particles onto a magneticrecording medium. By controlling the particle diameter of an oxideincorporated into a target for providing a magnetic layer, it has beenfound that abnormal discharge can be suppressed, and adhesion of oxideparticles onto a magnetic recording medium can be prevented.

[0014] In the first aspect of the present invention, a sputtering targetfor film formation of the magnetic layer of a magnetic recording mediumcomprising at least a nonmagnetic undercoat layer, a magnetic layer, anda protective layer laminated sequentially on a nonmagnetic substratecomprises a mixture of a metal and an oxide, wherein a particle diameterof the oxide being 10 μm or less.

[0015] Here, the particle diameter of the oxide may be 5 μm or less.

[0016] The mixture may comprise an alloy containing at least Co and Pt,and at least one oxide selected from the group consisting of oxides ofSi, Ti, Zr, Al and Cr.

[0017] In the second aspect of the present invention, a method forproducing a magnetic recording medium comprising at least a nonmagneticundercoat layer, a magnetic layer, and a protective layer laminatedsequentially on a nonmagnetic substrate, comprises the step of formingthe magnetic layer by RF sputtering of a sputtering target for themagnetic recording medium, wherein the sputtering target for themagnetic recording medium comprises a mixture of a metal and an oxide,and a particle diameter of the oxide in the sputtering target is 10 μmor less.

[0018] Here the particle diameter of the oxide in the sputtering targetis may be 5 μm or less.

[0019] The sputtering target may be a mixture comprising an alloycontaining at least Co and Pt, and at least one oxide selected from thegroup consisting of oxides of Si, Ti, Zr, Al and Cr.

[0020] In the third aspect of the present invention, a magneticrecording medium comprising at least a nonmagnetic undercoat layer, amagnetic layer, and a protective layer laminated sequentially on anonmagnetic substrate, where the magnetic layer is a granular magneticlayer having a structure in which crystal grains containing Co andhaving ferromagnetism are surrounded with oxide grain boundaries, themagnetic layer has been obtained by RF sputtering of a sputtering targetfor a magnetic recording medium, the sputtering target comprising amixture of a metal and an oxide, the oxide having a particle diameter of10 μm or less, and defects attributed to particles of the oxide andmeasuring 0.05 μm or more are not present on a surface of the magneticrecording medium.

[0021] According to the present invention, a magnetic layer is formed byRF sputtering of the above-described sputtering target, whereby agranular magnetic layer having excellent magnetic characteristics andelectromagnetic conversion characteristics can be formed by a simpleprocess. Furthermore, the invention is free from the problem of adhesionof oxide particles onto a magnetic recording medium.

[0022] The above and other objects, effects, features and advantages ofthe present invention will become more apparent from the followingdescription of embodiments thereof taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a schematic sectional view of a magnetic recordingmedium according to the present invention;

[0024]FIGS. 2A and 2B are SEM (scanning electron microscope) images ofthe surface of a target used, in which FIG. 2A shows the target with anSiO₂ particle diameter of 10 μm or less according to the presentinvention, while FIG. 2B shows the target with an SiO₂ particle diameterof more than 50 μm, as a comparative example;

[0025]FIG. 3 shows an SEM image of foreign objects adhering onto thesurface of a magnetic recording medium when a magnetic layer was formedusing the target with an SiO₂ particle diameter of more than 50 μm, thetarget shown in FIG. 2B; and

[0026]FIG. 4 shows the results of EDX (energy dispersive X-raydiffraction) analysis of foreign objects adhering onto the surface ofthe magnetic recording medium when the magnetic recording medium wasprepared using the target with an SiO₂ particle diameter of more than 50μm, the target shown in FIG. 2B.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0027] The present invention will now be described in more detail withreference to the accompanying drawings.

[0028] A sputtering target according to the present invention is usedfor film formation of a magnetic layer of a magnetic recording medium asan embodiment of the present invention as described in FIG. 1. FIG. 1 isa schematic sectional view of a magnetic recording medium showing anembodiment of the present invention. The numeral 1 denotes a nonmagneticsubstrate, 2 a nonmagnetic undercoat layer, 3 a magnetic layer,especially, a granular magnetic layer, 4 a protective layer, and 5 aliquid lubricant layer.

[0029] The magnetic recording medium shown in FIG. 1 is obtained by amanufacturing method having the step of providing the nonmagneticundercoat layer 2 on the nonmagnetic substrate 1, the step of providingthe magnetic layer 3 on the nonmagnetic undercoat layer 2, the step ofproviding the protective layer 4 on the magnetic layer 3, and the stepof providing the liquid lubricant layer 5 on the protective layer 4.

[0030] As the nonmagnetic substrate 1, there can be used, for example, aNiP-plated Al alloy substrate, glass substrate, or plastic substrate.The step of providing the nonmagnetic undercoat layer 2 on thenonmagnetic substrate 1 is performed by a conventional method such assputtering. As the nonmagnetic undercoat layer 2, there can be used, forexample, Cr or Cr alloy.

[0031] The step of providing the magnetic layer 3 on the nonmagneticundercoat layer 2 is a step for RF sputtering of a target which is amixture of a metal, especially, a ferromagnetic metal, and an oxide. Inthe present invention, it is important that the particle diameter of theoxide in the target be 10 μm or less in order to suppress the adhesionof oxide particles onto the magnetic recording medium. Preferably, theparticle diameter of the oxide in the target is made 5 μm or less,whereby the adhesion of oxide particles can be suppressed completely.

[0032] The target available on this occasion may be a target of acomposition which gives desired magnetic characteristics andelectromagnetic conversion characteristics. Preferably, the target isone having the ferromagnetic metal portion which is an alloy at leastcontaining Co and Pt. The oxide added to the alloy is preferably atleast one oxide selected from the group consisting of oxides of Si, Ti,Zr, Al and Cr.

[0033] Then, the protective layer 4 is formed on the magnetic layer 3.The protective layer 4 can be formed by conventional method such assputtering and CVD. For example, a layer of carbon 3 is used asprotective layer 4. Optionally, the liquid lubricant layer 5 consistingof lubricant such as perfluoropolyether is formed on the protectivelayer 4 by dipping.

[0034] In above described matter, magnetic recording medium of thepresent invention is provided.

[0035] In the present invention, the particle diameter of the oxideadded to the target used when providing the magnetic layer 3 is madesmall, as described earlier. By this measure, the degree of charging ofthe oxide observed particularly at the start of sputter discharginglowers to a level at which no abnormal discharge occurs. Thus, the oxideparticles do not adhere any more onto the surface of the magnetic layerduring the formation of the magnetic layer. That is, the magneticrecording medium having the magnetic layer formed with the use of thetarget in the present invention is free from defects occurring on themedium owing to the oxide particles and measuring 0.05 μm or more.

EXAMPLES Example 1

[0036] A plurality of smooth-surface crystallized glass substrates(crystallized glass substrates TS-10 of (Kabushiki Kaisha) Ohara Inc.,3.5 inches (88.9 mm), 0.8 mm thick) as a nonmagnetic substrate 1 werereadied for use. These glass substrates were washed, and then introducedinto a sputtering device to form a pure Cr undercoat layer 2 with a filmthickness of 15 nm on each of the glass substrates.

[0037] Then, a (CoCr₁₂Pt₁₂)₉₅—(SiO₂)₅ target was subjected to RFsputtering at an Ar gas pressure of 5 mTorr to form a granular magneticlayer 3 with a film thickness of 20 nm. The target used on this occasionfor film formation of the magnetic layer was a target containing SiO₂particles with varying particle diameter.

[0038] On the magnetic layer, a carbon protective layer 4 with a filmthickness of 10 nm was laminated, and the laminate was taken out of thevacuum. Then, a liquid lubricant was coated to a thickness of 1.5 nm toform a liquid lubricant layer 5. In this manner, a magnetic recordingmedium configured as shown in FIG. 1 was prepared. Heating of thesubstrate prior to film formation had not been performed.

[0039]FIGS. 2A and 2B are SEM (scanning electron microscope) images ofthe surface of the target used, in which FIG. 2A shows the surface ofthe target containing SiO₂ particles having a particle diameter of 5 μmor less, while FIG. 2B shows the surface of the target containing SiO₂particles having a particle diameter of more than 50 μm. These targetshave an equal SiO₂ content of 5%, but the target of A and the target ofB are found to be greatly different in the particle diameter of SiO₂.

[0040]FIG. 3 shows an SEM image of foreign objects adhering onto thesurface of the magnetic recording medium when the magnetic recordingmedium was prepared using the target containing SiO₂ particles having aparticle diameter of more than 50 μm, the target shown in FIG. 2B. FIG.4 shows the results of EDX (energy dispersive X-ray diffraction)analysis of the foreign objects shown in FIG. 3. The foreign objects arefound to be SiO_(x), and have a size of about 10 μm.

[0041] In each of the magnetic recording media obtained in theabove-described manner, the number of the foreign objects adhering ontothe magnetic recording medium was measured with an optical appearancetest device. Table 1 shows the particle diameter of SiO₂ contained inthe target used when forming the magnetic layer, and the number of theforeign objects adhering onto the magnetic recording medium as theaverage value of the ten magnetic recording media. For comparison, theresults obtained with the use of the target containing no SiO₂ are alsoshown. TABLE 1 Number of foreign objects Particle diameter adhering tothe medium (number of SiO₂ in the target per medium) Comparison: no SiO₂1 or less 5 μm or less 1 or less 10 μm or less 2.0 15 μm or less 11 20μm or less 24 100 μm or less 152

[0042] As shown in Table 1, when the particle diameter of SiO₂ is morethan 10 μm, many foreign objects are detected on the magnetic recordingmedium. At a particle diameter of 10 μm or less, the number of foreignobjects sharply decreases, and few foreign objects are detected at aparticle diameter of 5 μm or less. Thus, controlling the particlediameter of SiO₂ added to the target is found to prevent the adhesion offoreign objects onto the medium.

Example 2

[0043] Magnetic recording media were prepared in exactly the same manneras described in Example 1, except that a (CoCr₇Pt₁₂)₉₇—(Cr₂O₃)₃ targethaving different particle diameters was used as a target for formationof a magnetic layer.

[0044] In each of the magnetic recording media obtained in theabove-described manner, the number of foreign objects adhering onto themagnetic recording medium was measured with an optical appearance testdevice. Table 2 shows the particle diameter of Cr₂O₃ contained in thetarget used when forming the magnetic layer, and the number of theforeign objects adhering onto the magnetic recording medium as theaverage value of the ten magnetic recording media. For comparison, theresults obtained with the use of the target containing no Cr₂O₃ are alsoshown. TABLE 2 Particle diameter Number of foreign objects of Cr₂O₃ inthe adhering to the medium (number target per medium) Comparison: no 1or less Cr₂O₃ 5 μm or less 1.2 10 μm or less 1.9 20 μm or less 19 100 μmor less 255

[0045] Table 2 shows that as in the case of the SiO₂ containing targetshown in Table 1, when the particle diameter of Cr₂O₃ is more than 10μm, many foreign objects are detected on the magnetic recording medium.At a particle diameter of 10 μm or less, the number of foreign objectssharply decreases, and few foreign objects are detected at a particlediameter of 5 μm or less.

[0046] As discussed above, in connection with the target incorporatingthe oxide in the ferromagnetic alloy, the target used when forming agranular magnetic layer, the particle diameter of the oxide iscontrolled to 10 μm or less, preferably 5 μm or less, according to thepresent invention. By this measure, abnormal discharge during sputteringcan be suppressed, and adhesion of oxide particles to the medium can beinhibited. The so controlled target is RF sputtered to form a granularmagnetic layer. As a result, an excellent magnetic recording medium freefrom defects or corrosion can be produced by a simple process.

[0047] The present invention has been described in detail with respectto preferred embodiments, and it will now be apparent from the foregoingto those skilled in the art that changes and modifications may be madewithout departing from the invention in its broader aspects, and it isthe intention, therefore, in the appended claims to cover all suchchanges and modifications as fall within the true spirit of theinvention.

What is claimed is:
 1. A sputtering target for film formation of themagnetic layer of a magnetic recording medium comprising at least anonmagnetic undercoat layer, a magnetic layer, and a protective layerlaminated sequentially on a nonmagnetic substrate comprising: a mixtureof a metal and an oxide, wherein a particle diameter of said oxide being10 μm or less.
 2. The sputtering target as claimed in claim 1 , whereinthe particle diameter of said oxide is 5 μm or less.
 3. The sputteringtarget as claimed in claim 1 , wherein said mixture comprises an alloycontaining at least Co and Pt, and at least one oxide selected from thegroup consisting of oxides of Si, Ti, Zr, Al and Cr.
 4. The sputteringtarget as claimed in claim 2 , wherein said mixture comprises an alloycontaining at least Co and Pt, and at least one oxide selected from thegroup consisting of oxides of Si, Ti, Zr, Al and Cr.
 5. A method forproducing a magnetic recording medium comprising at least a nonmagneticundercoat layer, a magnetic layer, and a protective layer laminatedsequentially on a nonmagnetic substrate, comprising the step of; formingsaid magnetic layer by RF sputtering of a sputtering target for themagnetic recording medium, and wherein said sputtering target for themagnetic recording medium comprises a mixture of a metal and an oxide,and a particle diameter of said oxide in the sputtering target is 10 μmor less.
 6. The method for producing a magnetic recording medium asclaimed in claim 5 , wherein the particle diameter of said oxide in thesputtering target is 5 μm or less.
 7. The method for producing amagnetic recording medium as claimed in claim 5 , wherein saidsputtering target is a mixture comprising an alloy containing at leastCo and Pt, and at least one oxide selected from the group consisting ofoxides of Si, Ti, Zr, Al and Cr.
 8. The method for producing a magneticrecording medium as claimed in claim 6 , wherein said sputtering targetis a mixture comprising an alloy containing at least Co and Pt, and atleast one oxide selected from the group consisting of oxides of Si, Ti,Zr, Al and Cr.
 9. A magnetic recording medium comprising at least anonmagnetic undercoat layer, a magnetic layer, and a protective layerlaminated sequentially on a nonmagnetic substrate, wherein the magneticlayer is a granular magnetic layer having a structure in which crystalgrains containing Co and having ferromagnetism are surrounded with oxidegrain boundaries, the magnetic layer has been obtained by RF sputteringof a sputtering target for a magnetic recording medium, the sputteringtarget comprising a mixture of a metal and an oxide, the oxide having aparticle diameter of 10 μm or less, and defects attributed to particlesof the oxide and measuring 0.05 μm or more are not present on a surfaceof the magnetic recording medium.